Allow customizing merge logic of global and local entries

[bdarcus]

I pulled this out as a separate issue, because I think we don't want to lose track of it.

I'm not sure super sure on the details, but the use case is better handling cases where users are mixing global and local bib files, and there's duplicate entries, where the entry keys may or may not be the same.

Right now, entry merging is neither smart nor customizable. But it is fast.

See also #695 (comment).

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... this is also tied to broader issues, like how citar merges multiple bibliographies. In theory, there should not really be duplicate entries with different keys, but we currently don't check for that.

I could imagine a feature where we did, and we have some customization option on what to do with duplicates (and a related feature we may have previously discussed where one could import local entries into a global file).

Also related, potentially: #645.

Originally posted by @bdarcus in #695 (comment)